A network of the 2nd generation mobile communications technology (2nd Generation, 2G) or 3rd generation mobile communications technology (3rd Generation, 3G for short) achieves comprehensive coverage after being deployed for many years. At the same time, with the development of the network technologies, some PS domain-only networks that only provide packet switches (Packet Switch, PS for short), for example, a Long Term Evolution (Long Term Evolution, LTE for short) network, gradually cover some urban areas and areas with high traffic in the initial stage of network construction. Therefore, these PS domain-only networks and the 2G/3G network coexist in a current communications network.
For different communications networks, a voice service has different bearing mechanisms. Specifically, in the 2G/3G network, a voice call supports a conversational-class service borne by a circuit switched (Circuit Switched, CS for short) domain; whereas in a PS domain network, a voice call supports a conversational-class service borne by a packet switched (Packet Switched, PS for short) domain, which is also referred to as a voice over Internet Protocol (Voice over Internet Protocol, VOIP for short) service, where quality of service is ensured by an IP multimedia subsystem (IP Multimedia Subsystem, IMS for short).
Compared with a CS domain network, the PS domain network is capable of providing a voice service with a higher rate and higher quality for a user and satisfying more data service requirements of the user. Therefore, to develop more voice users of the PS domain network, an operator usually sets a priority of the PS domain higher than that of the CS domain to enable a voice user to preferably camp on the PS domain network. In the situation that the CS domain network and the PS domain network coexist, when a user equipment (User Equipment, UE for short) initiates a voice call in the CS domain of the 2G or 3G network and then moves to an area that is also covered by the PS domain network, based on the requirement for higher quality of a voice service or considering the restriction of a high priority, the UE needs to hand over the voice call from the CS domain to the PS domain of the LTE/HSPA network, that is, performing a procedure for processing reverse single radio voice call continuity (Single Radio Voice Call Continuity, SRVCC for short). In the handover procedure, for the voice call, voice interruption delay caused by the handover cannot exceed 300 ms. Otherwise, user experience is poor.
The existing reverse SRVCC processing is generally implemented by using the following two solutions. The first solution is that when performing the reverse SRVCC processing, the UE enables the voice service to exit from the 2G/3G CS domain network first and then access the LTE network, and initiates an access transfer processing procedure. In this solution, for a network access procedure, the UE enables the voice service to access the LTE network quickly, which avoids large voice interruption delay and does not affect user experience of voice continuity. For the access transfer, however, duration varies according to different application scenarios and the shortest needed duration is highly greater than the needed minimum value 300 ms of interruption delay during the voice handover, which severely affects voice continuity during the reverse SRVCC handover and causes poor quality of service.
The second reverse SRVCC processing solution is that when performing the reverse SRVCC processing, the UE hands over, in the 2G/3G network, a bearer of the voice service from the CS domain to the PS domain first, that is, performing an access transfer in the 2G/3G network first, and then performs access to the LTE network to completely hand over the voice service to the PS domain of the LTE network. In this solution, although the service continuity of the voice service of the UE between the LTE PS domain network and the 2G/3G CS domain network can be ensured by using the transition of a PS domain voice service of the 2G/3G network, the PS domain voice service supported by a 2G/3G PS domain network is only capable of providing services of low quality for the UE, and therefore the user cannot enjoy a voice service with good quality for a long period of time.